This invention relates in general to cast vehicle wheels and in particular to a method and an apparatus for forming lightener pockets in such wheels.
Cast wheels formed from light weight metals and metal alloys are replacing steel wheels on an increasing number of vehicles. Such cast wheels provide both a reduction in weight from steel wheels and an attractive appearance. Referring now to the drawings, there is illustrated in FIG. 1 a perspective view of a typical cast vehicle wheel, indicated generally at 10, which is formed in accordance with the prior art. The prior art wheel 10 includes an annular wheel rim 11 which is adapted to carry a vehicle tire (not shown). The wheel rim 11 has an axial inner end 12 which is adjacent to the vehicle (not shown) when the wheel 10 is mounted thereon and an axial outer end 13 opposite from the inner end 12.
The wheel 10 further includes a circular wheel disk 14 formed across the outer end 13 of the wheel rim 11. The wheel disk 14 has an outer surface 15 which is visible when the wheel 10 is mounted upon the vehicle. Accordingly, the outer disk surface 15 is typically machined to a smooth finish and/or decorated. As shown in the sectional view of FIG. 2, the junction of the wheel disk 14 and the rim 11 forms a sidewall 17 which is thicker than either the wheel disk 14 or the rim 11. The wheel disk 14 includes a plurality of radial spokes 18 connecting the sidewall 17 to a central wheel hub 19. The spokes 18 define a plurality of openings 20 formed through the wheel disk 14. The openings 20 allow a flow of cooling air to the vehicle brakes while reducing the weight of the wheel 10.
The hub 19 includes a recessed center portion 21 having a large central aperture 22 formed therethrough which can receive the end of a vehicle axle (not shown). The recessed center portion 21 also includes a plurality of smaller apertures 23 formed therethrough and spaced equally about the central aperture 22. These smaller apertures 23 receive threaded wheel lugs (not shown) for securing the wheel 10 upon a vehicle.
The wheel 10 is typically formed by gravity feeding or pressure injecting a molten metal, such as aluminum, magnesium or titanium, into a multi-piece wheel mold. A sectional view of a typical prior art multi-piece mold 30 for casting the wheel 10 is shown in FIG. 3. The individual pieces of the mold 30 are usually formed from cast iron or high carbon steel.
The mold 30 includes a bottom segment 31 which supports the other mold pieces. Two or more movable side segments 32 and 33 are carried by the bottom segment 31. The side segments 32 and 33 are moved in a radial direction between extended and retracted positions by a mechanism (not shown) that is conventional in the art. The side segments 32 and 33 are shown in the extended position in FIG. 3, with the side segments 32 and 33 together. When moved to the retracted position (not shown), the side segments 32 and 33 are separated and moved apart from each other.
A movable cup-shaped top segment 34 having a annular-shaped base 35 is disposed within the mold side segments 32 and 33 with the base 35 contacting the top surface of the bottom segment 31, as shown to the left of FIG. 3. The bottom surface of the top segment base 35 includes a plurality of radial grooves 36 formed thereacross in extended portions 36, one of which is shown to the right of FIG. 3. The radial grooves 36 are spaced equally apart about the base 35. A hub riser segment 37 is secured to the top of the top segment base 35. A central vent 37A is formed through the top of the hub riser segment 37 to allow gases to escape from the mold 30 during casting operations. A segment plug 38 having a plurality of slots 38A for venting gases is disposed in the central vent 37A. A handle 38 B is attached to the top of the segment plug 38. A mechanism (not shown), that is conventional in the art, raises and lowers the top segment 34 and hub riser segment 37 along a central axis 39.
The mold segments 31, 32, 33, 34, 36 and 37, upon assembly, define a mold cavity 40 wherein the wheel 10 is cast. The mold cavity 40 includes an annular rim cavity 41 for casting the wheel rim 11 and a disk cavity 42 for casting the wheel disk 14. The disk cavity 42 includes radial spoke cavities 43 defined by the radial grooves 36 in the top segment base 35. An annular sidewall cavity 44 for casting the wheel sidewall 17 joins the rim cavity 41 to the disk cavity 42. An annular rim riser cavity 45 is formed adjacent to the rim cavity 41. Similarly, a cylindrical hub riser cavity 46 is defined by the hub riser segment 37 adjacent to the inner center of the disk cavity 42.
To cast a wheel 10, molten metal is fed by a conventional method, such as gravity or under pressure, into the mold cavity 40 through a sprue (not shown). The molten metal flows into the disk, spoke, sidewall and rim cavities 42, 43, 44 and 41, and then fills the rim and hub riser cavities 45 and 46. As the metal contained in the mold cavity 40 cools and contracts molten metal flows from the riser cavities 45 and 46 back into the mold cavity 40 to fill any voids formed by the contraction.
Once the metal has cooled sufficiently, the top segment 34 is raised and the side segments 32 and 33 moved to their retracted position to allow removal of the wheel casting from the mold 30. Additionally, when needed, an operator grasps the handle 38A and manually removes the segment plug 38 to clean any excess casting metal from the segment plug slots 38A.
Following removal from the mold 30, the wheel casting is subjected to conventional machining operations to form the finished wheel 10.
As described above, the sidewall 17 of the cast wheel 10 is typically thickest at its junctions with the ends of the wheel spokes 18, adding weight to the wheel 10. The corresponding thickness of the sidewall mold cavity 43 holds a large volume of molten metal which can take longer to cool than the molten metal contained in the rim and wheel disk cavities 41 and 42. When this happens, the wheel rim 11 and disk 14 can solidify before the sidewall 17. Molten metal in the rim and hub riser cavities 44 and 45 is then prevented from flowing into the sidewall cavity 43 as the metal therein contracts. As a result, voids can be formed in the sidewall 17, reducing the strength of the wheel 10 and potentially marring the wheel's appearance.
It is known in the art to form recesses, or lightener pockets, in the outer surface of the sidewall 17 adjacent to the ends of the wheel spokes 18. Such a cast wheel is described in U.S. Pat. No. 4,861,113 to Imamura, et al. The lightener pockets reduce the thickness of the wheel sidewall 17 causing the molten metal in the sidewall cavity 43 to cool more rapidly than in the rim and disk cavities 41 and 42. As a result, molten metal in the hub and rim riser cavities 44 and 45 can flow into the sidewall cavity 43 as the metal therein contracts. This substantially reduces the possibility of voids being formed in the wheel sidewall 17 as the metal solidifies. Additionally, the lightener pockets reduce the weight of the wheel.
It is known in the art to use loose core pieces to form recesses in cast parts. The loose pieces are removed from the recesses after the casting is removed from the mold. When light weight metals, such as aluminum, are used to cast such parts, the metal surrounding the loose core pieces shrinks as it cools, making it difficult to remove the loose pieces from the recesses. Thus, a separate machine is usually required to pull the loose pieces from the recesses. As an alternative, it is also known to machine the recesses into the casting. However, machining is time consuming and expensive.